1. Field of the Invention
The present invention relates to a magnetic element which exhibits a rapid change in magnetization with a change in an externally applied magnetic filed.
2. Description of the Related Art
There are many devices which utilize the magnetization behavior of a magnetic material. In addition to devices which exhibit a continuous response to a change in an external magnetic field such as a magnetic induction type magnetic head, magnetic materials which exhibit a rapid magnetic reversal and a discontinuous response when the intensity of the applied magnetic field exceeds a predetermined value have recently been employed. When a pickup coil is disposed in the vicinity of such a magnetic material, a steep voltage pulse can be produced in the coil upon a discontinuous magnetic reversal of the magnetic material. The use of such a magnetic element can provide a simplified apparatus which is widely applicable to the measurement of magnetic fields such as the earth's magnetic field, rotational speed, flow rate, etc.
Furthermore, in recent years, electronic article surveillance systems or identification systems for preventing the theft of merchandises or for rapidly processing the flow of materials have become more widely used. These devices employ identifying markers such as a transmitting circuit, an LC resonance circuit, a magnetostrictive vibrating material and a high magnetic permeability material, as well as the above-described magnetic material which exhibits a discontinuous magnetic reversal. For example, U.S. Pat. Nos. 4,660,025, 4,686,516 and 4,797,658 disclose a system employing a marker made of a fine amorphous Fe based alloy wire. The magnetization of the foregoing fine metal wire material is extremely stable in the longitudinal direction and thus exhibits a very sudden 180.degree. magnetic reversal when the magnetic field reaches a predetermined magnitude. This characteristic is often called a large Barkhausen discontinuity. When the intensity of an alternating magnetic field which has been transmitted as an inquiry signal in a monitor zone reaches a critical value, the fine metal wire exhibits a discontinuous magnetic reversal, thereby causing a detection coil to produce a steep pulse voltage. The waveform of the pulse voltage thus produced is then subjected to a frequency analysis in which the intensity and proportion of high harmonics are determined to identify the marker or to judge if it is necessary to sound an alarm. This system is advantageous in that the marker is inexpensive and provides an identifying capacity higher than that of other systems.
Magnetic materials have been found which exhibit a discontinuous magnetization response besides the foregoing fine amorphous metal wire. For example, U.S. Pat. Nos. 4,980,670 and 5,313,192 disclose a material obtained by annealing a slender amorphous metal ribbon in a magnetic field. Furthermore, U.S. Pat. No. 5,181,020 discloses a thin film having a strong uniaxial magnetic anisotropy formed on a polymer substrate such as a plastic film which exhibits a discontinuous magnetic reversal. This material exhibits excellent rectangular hysteresis characteristics similar to the fine metal wire.
In order to practically use a thin film having a strong uniaxial magnetic anisotropy formed on a plastic polymer substrate as a magnetic element (e.g., as a sensor and marker), the thin film must be cut into a desired shape together with the substrate. However, when the laminate is mechanically cut by a cutter, scissors or the like, unnecessary stress is applied to the thin film even if a relatively sharp blade is used. This stress occasionally disturbs the uniaxial magnetic anisotropy of the thin film. Accordingly, the resulting magnetic element is disadvantageous in that its magnetic characteristics can vary widely.